Coherent sources of infrared radiation have been used in the past for a variety of purposes, not the least of which has been in infrared countermeasure systems. Typically, for laser range finding and for jamming purposes, laser systems have been provided which project 1.5–3 micron radiation towards a target. In some cases, the targets are the seekers used in shoulder-fired or other types of missiles.
There is, however, a need to provide longer wavelength infrared radiation for two distinct purposes. The first is to be able to detect and countermeasure missile seekers which are now utilizing longer wavelengths in the mid infrared above 5 microns all the way out to the long infrared at 20 microns.
Additionally, longer wavelength infrared sources are useful for spectrum analysis purposes and also to be able to detect pollutants or chemical warfare agents in the atmosphere. Specifically, it is important to be able to recognize the existence of gaseous chemical agents dispersed in the atmosphere, and for this purpose mid to long wavelength infrared radiation is required.
In the past there have been multitude ways to generate longer IR wavelengths. To date CO2 lasers, some diode lasers and quantum well lasers have been able to produce infrared radiation in the 10 micron range. However, these lasers are not tunable over a wide bandwidth. For instance, CO2 lasers are only tunable between 9–11 microns.
More recently, it has been found that cadmium germanium arsenide optical parametric oscillators can be utilized as a wideband mid to long wavelength infrared source, assuming enough power could be used to pump the optical parametric oscillator at 5 to 6 microns. This, however, has been difficult because pumping sources for such an optical parametric oscillator have not had sufficiently narrow line widths so as to be able to concentrate enough energy into the cadmium germanium arsenide nonlinear crystal.
If one were able to produce a narrow enough line width with a 5 micron pump, then one would be able to generate a tunable continuum of wavelengths from 5 microns out to 20 microns with the cadmium germanium arsenide optical parametric oscillator. Note that the tuning of such an optical parametric oscillator can be achieved either by angle tuning or temperature variation so as to have a true tunable broadband source.
There is therefore a problem insofar as being able to find a way to take a 5 micron laser source and make it narrow enough to pump the cadmium germanium arsenide optical parametric oscillator. The ability to do so would permit not only the countermeasure application but also use in spectroscopy and gas detection.
It will be noted that prior longer wavelength IR lasers such as the aforementioned CO2 laser are tunable by gratings to permit tuning between 9 and 11 microns. However, this is a narrow window and if one were looking for a broadbanded IR source, carbon dioxide lasers are not particularly useful.
As mentioned hereinbefore, with cadmium germanium arsenide optical parametric oscillators, prior efforts to efficiently pump such an optical parametric oscillator met with failure due to the fact that the line width of the 5 micron pumping radiation was too broad.
More particularly and by way of further background, zinc germanium phosphide (ZnGeP2, or ZGP) is the only mature nonlinear crystal capable of generating high power, tunable optical parametric oscillator (OPO) output beyond 4 microns. A high nonlinear coefficient (76 pm/V), high thermal conductivity (0.36 W/cm K), linear and nonlinear absorption below 1.8 μm, and exceptional transparency beyond 1.9 μm, make it the material of choice for 2-micron pumped OPOs as is disclosed in P. G. Schunemann, “Nonlinear crystals provide high power for the mid-IR,” Laser Focus World 35, 85–88 (1999). While several other materials have been used to demonstrate tunable output in the mid-infrared, few are suitable for high power operation beyond 4 microns, either due to high absorption losses or poor thermal conductivity.
As to far-infrared OPOs, cadmium germanium arsenide (CdGeAs2, or CGA) is another nonlinear optical material currently under development. It has one of the highest known nonlinear coefficients (216 pm/V) and is useful for generation of tunable OPO output from 5–20 μm as is disclosed in P. G. Schunemann, “Nonlinear crystals provide high power for the mid-IR,” Laser Focus World 35, 85–88 (1999), but requires a pump source >4.6 um to avoid two-photon pump absorption. One of the few usefully intense pump sources is the idler from a ZGP OPO, but the spectral characteristics are generally too broad to make it an effective pump source.
Narrow mid-IR line width has been demonstrated in Type I ZGP OPOs by use of a grating and etalon tuning elements F. Ganikhov, T. Caughey, and K. L. Vodopyanov, “Narrow-linewidth middle-infrared ZGP optical parametric oscillator,” J. Opt. Soc. Am. B 18, 818–822 (2001), but such a geometry is not conducive to high power, low threshold operation. Type II operation with a 2 μm pump, which has much narrower OPO acceptance bandwidth than Type I, cannot phase match idler wavelengths below 5.5 μm. The spectral output can be narrowed by use of a singly resonant oscillator (SRO) configuration, but this increases threshold beyond practically acceptable levels. Additionally, one may line-narrow the pump source, running on a single longitudinal mode, but this is often impractical from an engineering and fieldability standpoint.